The present invention relates to a pneumatic braking system braking force modulator. The modulator, when coupled with suitable devices for sensing the state of rotation of the wheels of a vehicle, forms a complete system for preventing wheels from becoming locked when brake forces are applied which exceed the limit corresponding to the actual state of friction existing at a certain moment between the wheels and the road surface.
In such complete so-called "anti-lock" or "anti-skid" systems the basic function is such that when the sensor monitoring the state of the rotation of the vehicle wheel during braking detects that the wheel tends to be retarded beyond a certain limit, the system signals the braking force modulator to prevent additional accumulation of braking force and possibly also to reduce already applied braking force. The retardation of the vehicle wheel is thus reduced and transformed into an acceleration. When the vehicle wheel has reached a speed approaching the speed of a non-braked wheel at the actual vehicle speed, the system responsive to the wheel sensor may signal the braking force modulator to increase the braking force or to hold it on a constant level.
As the present invention only refers to a braking force modulator, there is no reason here to deal in detail with wheel sensors which are available in many well-known constructions, both mechanical and electronic, representing various control philosophies. Some of these sensors produce only two orders, viz. (1) to prevent additional braking force accumulation and reduce already applied braking force or (2) to increase the braking force. As previously intimated, sensors are available which in addition to the above-mentioned two orders also produce a third order to hold the braking force at a constant level.
It appears from the above explanations that a universally useful braking force modulator, that is a modulator which may be used in combination with any type of wheel sensor known so far, must fulfill a requirement to be able to receive and execute four orders, namely:
1. Bar additional accumulation of braking force. PA1 2. Reduce already applied braking force. PA1 3. Increase braking force. PA1 4. Hold braking force at a constant level.
Preferably each wheel provided with a brake, which may be a driven wheel on a pulling vehicle or a freely rotating wheel on an attached trailing vehicle, forms a unit with a sensor and other equipment for preventing locking of the brake. For purposes of simplifying and clarifying the following discussion, this invention will hereinafter be described by reference to a single wheel provided with a brake. However, the expert in the applicable field will realize that other embodiments are possible. For example an embodiment may be mentioned in which the braking force on all brakes on a common shaft are controlled identically by a common system.
Lock-preventing systems for pneumatic brakes for vehicles are known in which the sensor signals actuate various valve arrangements such that the compressed air used for brake application proper is discharged completely or partly whenever a sensor signal ordering the braking force to be reduced is produced. Thus, this means that whenever the lock-preventing system comes into action, compressed air is released into the environment. During a braking process controlled by such a system, a series of such air discharges occurs in alternation with supply of air from the compressor or air-supply tank of the vehicle. The compressor is required incessantly to provide compensation for these pressure releases and, when the pressure releases are numerous and large during a comparatively short time, the compressor will not be able to supply the necessary air flow.
Thus the operation of such a system involves large consumption of compressed air and during repeated powerful braking on a slippery support this may cause the pressure of the pneumatic system to fall below the level at which the emergency or parking brakes will be actuated.
As air is compressible, there are always certain time lags required to build up or release pressure. The amount of available pressurized air is determined by the compressor and the tank, and it is not permissible, either economically or in respect to space, to dimension this equipment to supply the demands which may be made on some isolated occasions. Discharge and replenishment may be accelerated by increasing valve areas, which yields other problems such as bad sealing, long operation times, control difficulties and the like. In practice, lesser valve areas are still preferred and attempts at acceptable function with reasonable consumption of pressurized air are made by limiting the working frequency of the system.
Due to the problems discussed in the preceding paragraph such a conventional anti-lock system for pneumatic brakes functions slowly, often at a frequency between 1 and 2 Hz for the control cycles, while at the same time the consumption of pressurized air is so large that the pressure thereof after 10 to 15 seconds has fallen below a useful value. The driver experiences the braking as jerky and it is not unusual that the driver is forced to compensate for braking variations in the two forward wheels by virtually steering against braking action which gives neither the impression nor the effect of a fully reliable braking system. In many vehicles this renders individual control of the forward wheel brakes impossible.
U.S. Pat. No. 4,166,657 discloses a device for preventing pneumatically braked vehicle wheels from locking during braking. The basic principle of the system disclosed is that upon receipt of a sensor signal a counterforce is applied to a mechanical link in the braking system for reducing the braking force. The counterforce is provided by a hydraulic system comprising a tank for hydraulic fluid, a hydraulic pump driven by an electric motor, a valve which is normally open but which is closed upon application of a sensor signal, and a hydraulic cylinder which is continuously connected to the mechanical system actuating the brakes and thus moves every time braking is performed. The cylinder action causes hydraulic fluid to be pressed from the cylinder into the hydraulic fluid tank when the brakes are applied and sucked back from the hydraulic fluid tank into the hydraulic cylinder when the brake is released. The valve controlled by the sensor signal is inserted into the conduit to the hydraulic cylinder and is open during normal braking so that the system does not produce any opposing forces during normal movements of the braking mechanism. When during a brake application the sensor emits a closing signal to the valve, hydraulic fluid flow from hydraulic cylinder to the hydraulic fluid tank is interrupted and brake application is blocked. If, in addition, the sensor emits an order to start the electric motor driving the hydraulic pump, hydraulic fluid is pumped from the hydraulic fluid pump to the hydraulic cylinder causing the brake to be released due to the fact that the force from the hydraulic cylinder overcomes the force from those elements in the conventional braking system which operate the brake. If the motor is stopped and the valve is opened the brake is again applied. It will be appreciated that by causing the sensor to emit various combinations of signals to the valve and the motor, the device according to U.S. Pat. No. 4,166,657 can be caused to perform all the four functions mentioned above.
While this prior art device is satisfactory in many cases, it has shown that very strong electric motors will be needed to drive the hydraulic pump in heavy vehicle applications where the braking forces are considerable. Energy required to start a pumping motor often exceed 1 kW and a mean effect of 350 W may be required during a braking process controlled by the system. Such modulators and the associated driving motors are large, clumsy and expensive in addition to consuming excessive power.
Studies of test results obtained from heavy vehicles provided with pneumatic brakes and anti-lock systems in which pressurized air is released have shown that variations in the pressure of the pressurized air during modulated braking practically never are greater than about 2 bar, independently of the state of the road and the weight of the vehicle. A pressurized air system for vehicles normally holds a pressure of about 7 bar during braking and the pressure variations during modulated braking may be between 7 and 5 bar when the road is good and the vehicle is fully loaded. Depending on the state of the road and the weight of the vehicle the range of values will drop to 2 to 1.5 bar or lower when the friction ratio between the vehicle wheel and the road surface is particularly low. However, under any conditions the difference between the higher and lower pressures in the range will be about 1.5 to 2 bar, and only in rare cases will it be higher.